The conventional Ball Grid Array packages (BGA) are always using printed circuit boards as chip carriers. In one kind of BGA packages, the active surfaces of chips face to the printed circuit boards, and the bonding wires pass through the slots of the printed circuit boards. This kind of BGA package is often called Window BGA, Chip-On-Board BGA, or Chip-On-Substrate BGA.
As shown in FIG. 1, a conventional BGA package 100 includes a printed circuit board 110, a chip 120, a die-attaching layer 130, a plurality of bonding wires 140, an encapsulant 150, and a plurality of solder balls 160. The printed circuit board 110 is made of BT which has a top surface 111, a bottom surface 112, a slot 113 penetrating the top surface 111 and the bottom surface 112, a plurality of bonding fingers, and a plurality of ball pads, not shown in the figure. A plurality of bonding pads 40 are formed on the active surface 121 of the chip 120. The active surface 121 of the chip 120 is attached to the top surface 111 of the printed circuit board 110 by the die-attaching layer 130 so that the bonding pads 122 are aligned in the slot 113. The bonding pads 122 of the chip 120 are electrically connected to the printed circuit board 110 by the bonding wires 140 through the slot 113. The encapsulant 150 is formed on the top surface 111 of the printed circuit board 110 as well as in the slot 113 to encapsulate the chip 120 and the bonding wires 140. The solder balls 160 are disposed on the bottom surface 112 of the printed circuit board 110 as external terminals. Since the active surface 121 of the chip 120 is close to the printed circuit board 110, the lengths of the bonding wires 140 can be shortened so that it can be implemented in the packages of high-frequency memories such as DDR II or other ASIC.
However, the cost of the printed circuit board is much higher than the one of a leadframe and the reliability against moisture is also poor. If a leadframe can be used as a chip carrier to assembly as a BGA package, the cost can be greatly reduced. Leadframe-based BGA packages using leadframes as chip carriers have been proposed in the field. After leadframe etching, a chip is disposed on top surfaces of the leads of the leadframe and is electrically connected to the leadframe by bonding wires. Then, encapsulant encapsulates the chip and the bonding wires with the bottom surfaces of the leads of the leadframe exposed, i.e., the surfaces without disposing the chip. Then, solder balls are placed on the exposed surfaces of the leads followed by singulation of the leadframe and the encapsulant to manufacture a plurality of leadframe-based BGA packages. However, the solder balls totally exposed outside the package without any protections leading to the dropping of the solder balls during shipping, handling, and storage. Moreover, the bonding interfaces between the solder balls and leads are planar leading to cracks during SMT processes and under operations, the stability of solder ball placement and the reliability of the packages require further improvement.
Furthermore, conventional leadframe-based BGA packages are also taught by Lin in U.S. Pat. No. 5,830,800 and King et al. in U.S. Pat. No. 5,677,566. In spite of the solder balls are partially embedded in the cavities of the encapsulant but still are bonded to flat ball-placement surfaces of the leadframe, therefore, the joint strengths of the solder balls are still weak leading to dropped balls due to the stresses caused by the encapsulant or dielectrics.